[const prop] Fix "alloc id without corresponding allocation" ICE

Fixes #66345

src/librustc_mir/transform/const_prop.rs

@@ -649,9 +649,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
     }
 
     fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
-        if self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2 {
-            return true;
-        } else if self.tcx.sess.opts.debugging_opts.mir_opt_level == 0 {
+        if self.tcx.sess.opts.debugging_opts.mir_opt_level == 0 {
             return false;
         }
 

src/test/mir-opt/const_prop/const_prop_fails_gracefully.rs

@@ -23,9 +23,9 @@ fn main() {
 // START rustc.main.ConstProp.after.mir
 //  bb0: {
 //      ...
-//      _4 = const Scalar(AllocId(1).0x0) : &i32;
-//      _3 = const Scalar(AllocId(1).0x0) : &i32;
-//      _2 = const Value(Scalar(AllocId(1).0x0)) : *const i32;
+//      _4 = const main::FOO;
+//      _3 = _4;
+//      _2 = move _3 as *const i32 (Misc);
 //      ...
 //      _1 = move _2 as usize (Misc);
 //      ...

src/test/mir-opt/const_prop/ref_deref.rs

@@ -14,7 +14,7 @@ fn main() {
 // START rustc.main.ConstProp.after.mir
 // bb0: {
 //     ...
-//     _2 = const Scalar(AllocId(0).0x0) : &i32;
+//     _2 = &(promoted[0]: i32);
 //     _1 = const 4i32;
 //     ...
 // }

src/test/mir-opt/const_prop/reify_fn_ptr.rs

@@ -16,7 +16,7 @@ fn main() {
 // START rustc.main.ConstProp.after.mir
 //  bb0: {
 //      ...
-//      _3 = const main;
+//      _3 = const main as fn() (Pointer(ReifyFnPointer));
 //      _2 = move _3 as usize (Misc);
 //      ...
 //      _1 = move _2 as *const fn() (Misc);

src/test/mir-opt/const_prop/slice_len.rs

@@ -24,8 +24,8 @@ fn main() {
 // START rustc.main.ConstProp.after.mir
 //  bb0: {
 //      ...
-//      _4 = const Scalar(AllocId(0).0x0) : &[u32; 3];
-//      _3 = const Scalar(AllocId(0).0x0) : &[u32; 3];
+//      _4 = &(promoted[0]: [u32; 3]);
+//      _3 = _4;
 //      _2 = move _3 as &[u32] (Pointer(Unsize));
 //      ...
 //      _6 = const 1usize;

src/test/ui/consts/issue-66345.rs

@@ -0,0 +1,24 @@
+// run-pass
+// compile-flags: -Z mir-opt-level=3
+
+// Checks that the compiler does not ICE when passing references to field of by-value struct
+// with -Z mir-opt-level=3
+
+fn do_nothing(_: &()) {}
+
+pub struct Foo {
+    bar: (),
+}
+
+pub fn by_value_1(foo: Foo) {
+    do_nothing(&foo.bar);
+}
+
+pub fn by_value_2<T>(foo: Foo) {
+    do_nothing(&foo.bar);
+}
+
+fn main() {
+    by_value_1(Foo { bar: () });
+    by_value_2::<()>(Foo { bar: () });
+}